I just cut one this week. 14 inches out of the center and 5 inches off the back. I've done a fair number of frame splices and have always done it the straight way, full penetration welds, and a fish plate on the inside with small fish plates on the bottom webs of the framerails. There's a lot of people on here who will disagree with my methods, but a large percentage of them have never cut a frame in their life and just repeat what they've read on the internet, so take their advice with that in mind. I learned to do it this way from an old truck mechanic as we built dump trucks.
I fit them with about a 1/16 gap between the beveled edges, tack and measure to verify that you have it all straight and aligned, then weld it up. After adding the doubler plates to the inside, I plug weld through whatever holes happen to be there on the outside of the rails also. You can cut your fish plates out of the sections of rails you removed. Most of the time I stick weld them rather than use my MIG. That's just a personal preference though. Whatever you're most comfortable with.
Don't get too wrapped up in all the hysteria. It's not gonna break and send your careening into a bus full of nuns on their way to the orphanage, with a trailer full of puppies behind them. Worst case, you would see some cracking. Keep in mind, the bed adds a bit of a truss factor too since it's bolted in front of and behind your splices.